The present invention, in some embodiments thereof, relates to fixing the position of an ultrasound transducer with respect to a subject and, more particularly, but not exclusively, to fixing the position of an ultrasound transducer within a garment affixed to the subject.
Ultrasound is a well matured medical imaging modality. It provides two-dimensional (2D) and/or three-dimensional (3D) anatomic information as well as a plurality of physiological and functional parameters at relatively high refresh rates, reaching an order of 100 frames per second for 2D imaging.
The imaging platforms are portable and reasonably priced. However, ultrasound imaging suffers from some disadvantages, chief of which are low image quality compared to other imaging modalities, for example Computed Tomography (CT) and Magnetic Resonance Imaging (MRI); and limited volume coverage.
Conventional phased array transducers have a limited field of view due to limitations at the beam steering end, i.e., due to off-broadside beam widening; the effective area of planar transducers decreases with the off-broadside angle, thus increasing the beam width.
The maximal beam penetration depth is also limited by signal attenuation within the tissue. Decreasing the transmission frequency reduces the attenuation and increases the penetration depth, but also worsens the spatial resolution.
One of the methods known in the art for addressing these issues is image registration and compounding. The term ‘registration’ relates to the process of finding a transformation that maps each point in one image or coordinate system to corresponding points in another image or coordinate system. The term ‘compounding’ relates to the combination of data from multiple registered images to produce one or more registered images.
The registration process may either be algorithm based, or sensor based.
In algorithm based registration, the sole input is a set of reconstructed imaging planes or volumes. For example as described by Foroughi P, Abolmaesumi P, Hashtrudi-Zaad K; “Intra-Subject Elastic Registration of 3D Ultrasound Images”; Medical Image Analysis 2006; 10:713-725.
In sensor based registration, data acquired by positioning and/or orientating sensors is also utilized. For example as taught in US Patent Application 2007/0081709; Apr. 12, 2007 by Warmath R J, Herline A J; “Method and Apparatus for Standardizing Ultrasonography Training Using Image to Physical Space Registration of Tomographic Volumes from Tracked Ultrasound”.
The documented advantages and features of ultrasound image registration and compounding include, for instance:                i. Field of view extension; for example as described by Poon T C, Rohling R N; “Three-Dimensional Extended Field-Of-View Ultrasound”; Ultrasound in Medicine and Biology 2006; 32:357-369.        ii. Freehand three-dimensional (3D) imaging, i.e., generation of 3D images using a manually moved 2D imaging probe; for example as described by Treece G M, Gee A H, Prager R W; “RF and Amplitude-Based Probe Pressure Correction for 3D Ultrasound”; Ultrasound in Medicine and Biology 2005; 31:493-503.        iii. Speckle noise reduction. Speckle is a result of the fact that the reflecting particles within tissues are much smaller than the wavelength used. The effect of speckle may be modeled as multiplicative noise. Speckle patterns are very sensitive to small changes in the relative location of the transducer and the tissue volume, and can be reduced by local averaging over several frames, taken at different times or probe positions/orientations. For example as described by Krücker J F, Meyer C R, LeCarpentier G L, Fowlkes J B, Carson P L; “3D Spatial Compounding of Ultrasound Images Using Image-Based Nonrigid Registration”; Ultrasound in Medicine and Biology 2000; 26:1475-1488.        iv. Minimization of shadowing artifacts. Shadowing is caused by tissues along the ultrasonic beam that have a high reflection and/or attenuation coefficient, so that the ultrasound energy reaching tissues further away from the transducer (along the ultrasonic beam) is significantly reduced. This differently affects ultrasound images acquired from different angles, and can therefore be mitigated by spatial compounding. For example, as described by Krücker J F, Meyer C R, LeCarpentier G L, Fowlkes J B, Carson P L; “3D Spatial Compounding Of Ultrasound Images Using Image-Based Nonrigid Registration”; Ultrasound in Medicine and Biology 2000; 26:1475-1488.        v. Spatial resolution enhancement, using datasets obtained at multiple spatial locations and/or orientations of the probe; for example as described by Yang Z, Tuthill T A, Raunig D L, Fox M D, Analoui M; “Pixel Compounding: Resolution-Enhanced Ultrasound Imaging for Quantitative Analysis”; Ultrasound in Medicine and Biology 2007; 33:1309-1319.        vi. Estimation of speed of sound factors within different tissues, using time delays measured at different directions; for example as taught in US Patent Application 2007/0167757; Jul. 19, 2007; Haimerl M; “Determining Speed-of-Sound Factors in Ultrasound Images of a Body”.        vii. Change estimation and regional motion evaluation, utilizing 2D or 3D imaging information acquired for the same tissue volume at several timeframes. In some cases, the registration may be performed globally, but local registration is usually required, tracking the location change over time for every small region, for example as described by optic-flow techniques. If the time difference between consecutive images is relatively short, applying cross-correlation functions to the local speckle pattern can allow accurate regional motion assessment. For example as taught in U.S. Pat. No. 5,876,342; Mar. 2, 1999; Chen J F, Weng L; “System and Method for 3-D Ultrasound Imaging and Motion Estimation”.        viii. Angle independent Doppler measurement. Standard ultrasound systems estimate flow velocity, for example blood flow velocity, using the Doppler Effect, which only provides information regarding the radial component of the velocity vector. When multiple receiving transducers are employed, placed at different angular locations with respect to the target volume, one can estimate the magnitude and orientation of the flow velocity vector. For example as taught in U.S. Pat. No. 5,409,010; Apr. 25, 1995; Beach K, Overbeck J; “Vector Doppler Medical Devices For Blood Velocity Studies”.        ix. Intra-operative guidance, by registration of intra-operative ultrasound images to images acquired beforehand by any modality; for example as described by Barratt D C, Penney G P, Chan C S K et al.; “Self-Calibrating 3D-Ultrasound-Based Bone Registration for Minimally Invasive Orthopedic Surgery”; IEEE Transactions on Medical Imaging 2006; 25:312-323.        
Some state-of-the-art 3D imaging probes with improved field of view have also been suggested, for instance:                i. As taught in International Patent Application WO2004/001447; Dec. 31, 2003; Poland and Sumanaweera et al.; “System and Method for Electronically Altering Ultrasound Scan Line Origin for a Three-Dimensional Ultrasound System”; and as taught in US Patent Application 2006/0078196; Apr. 13, 2006; Sumanaweera T S, Cai A H, Ustuner K F; “Distributed Apexes for 3-D Ultrasound Scan Geometry”; which describe 2D or multi-dimensional (MD) phased arrays which can adaptively generate scan lines apparently emanating from a location (“apex”) other than the geometric center of the transducer probe. Multiple apexes may be generated, allowing the optimization of the scanned volume to the transducer's characteristics.        ii. As taught in US Patent Application 2006/0173333; Aug. 3, 2006; Sudol W.; “Two-Dimensional Transducer Arrays for Improved Field of View”; where a similar concept is presented, wherein different groups of adjacent rows and/or columns of transmitting and/or receiving transducer components are activated at different times. Sudol also suggests the possibility of using a convex 2D array, as well as using two or more probes concurrently.        iii. As taught in US Patent Application 2007/0066902; Mar. 22, 2007; Wilser and Mohr; “Expandable Ultrasound Transducer Array”; describing a foldable transducer array, intended to be used within the subject's body. While folded, the transducer array has a smaller width or volume, for insertion into and withdrawal from, for example a hollow region within the subject. When unfolded, foldable transducer array provides a larger radiation surface.        
Another system and technique, which effectively improves image quality and increases image volume, is ultrasound computed tomography (UCT). UCT is founded on inverse problem concepts, similar to those used for X-ray CT. UCT has two basic implementations:                i. Reflection mode: In this case, the source, for example the transmitting array, and the detector, for example the receiving array; are on the same side of the subject or the target region. The transmitting array and the detector are rotated about a certain rotation axis, and in some cases also translated along that axis. In each geometric configuration, a short ultrasound pulse is transmitted, and the reflected echoes, resulting from discontinuities in the speed of sound within the medium, are measured as a function of time, which corresponds to the distance between the reflector and the transducer; for example as taught in US Patent Application 2006/0106307; May 18, 2006; Dione D P, Staib L H, Smith W; “Three-Dimensional Ultrasound Computed Tomography Imaging System”.        ii. Transmission mode: In this configuration, the source and the detector are placed on opposite sides of the subject or the target region, and are rotated and/or translated together; for example as taught in U.S. Pat. No. 4,509,368; Apr. 9, 1985; Whiting J F, Koch R H L; “Ultrasound Tomography”. For each beam, the time difference between the transmission of the pulse and its detection on the other side; for example as described by using rise-time detection or threshold detection on receive; provides information regarding the overall time of flight, which is inversely proportional to the average speed of sound along the beam. The power ratio between the transmitted pulse and the received pulse provides an estimate of the total signal attenuation along the beam.        
In some cases, multiple sources and/or detectors are used to reduce the overall scanning time.
Both UCT modes use circular or helical scanning of the subject. A slightly different geometry has been suggested by Li P C, Huang S W; “Ultrasound Tomography of the Breast Using Linear Arrays”; ICASSP 2005; V-989-V-992; who compressed a female breast between a linear transducer array and a reflective metal plate. Separate groups of transducer components are allocated for signal transmission and reception. The relative location of the selected groups with respect to the metal plate determines the path of the ultrasonic beam.
A variation of UCT, called ultrasound diffraction tomography (UDT), is based on measuring the forward scattered ultrasound field as a function of cross-range with respect to the incident wave. This technique also requires the utilization of more complex reconstruction methods; for example as described by Louis A K; “Medical Imaging: State of the Art and Future Development”; Inverse Problems 1992; 8:709-738.
Furthermore, ultrasound imaging has the potential to expand its clinical applications beyond its presently prevalent capabilities, and also provide tissue classification parameters. Elastography has been proposed as a way to achieve this goal; for example as described by Melodelima D, Bamber J C, Duck F A, Shipley J A, Xu L; “Elastography for Breast Cancer Diagnosis using Radiation Force: System Development and Performance Evaluation”; Ultrasound in Medicine and Biology 2006; 32:387-396. The term elastography encompasses a variety of techniques that can depict a mechanical response or property of tissues. In ultrasound, the elastography premise is built on two known facts:                i. There are significant differences between mechanical properties of several tissue components.        ii. The time-dependent information contained in the measured speckle patterns is sufficient to depict these differences following an external or internal mechanical stimulus. This stimulus may be generated, for example by applying an external pressure to the skin surface, or by vibrating a region at a low frequency.        
Ultrasound also has therapeutic applications, using high intensity focused ultrasound (HIFU) technologies, which increase the local temperature at a region near the focal point of a high energy ultrasound transducer, thus causing local tissue ablation; for example as taught in US Patent Application 2008/0051656; Feb. 28, 2008; Vaezy S, Chan A N, Fujimoto V Y, Moore D E, Martin R W; “Method for Using High Intensity Focused Ultrasound”.